1. Field of the Invention
The invention concerns a Metal Detector with multipolar windings, to be placed at the entrance of premises needing protection from the admission of armed persons and for other uses connected with the interception of metal objects.
2. Description of the Related Art
The current state of the art in the field of passageway Metal Detectors has emerged from the successive adoption of technology that has made it possible to progressively improve them in terms of both their uniformity of detection, as well as their compatibility with their immediate environment which could be the cause of electromagnetic disturbance or interference.
Such development has meant that makers have concentrated more and more on multipolar transducers and windings that offer the best compatibility with metal structures and surrounding sources of electric interference.
Moreover, Metal Detector apparatus has been developed that is equipped with one or more transmitters and/or one or more field receivers, wherein the transmitter-receiver pairs are made to work simultaneously by some manufacturers, and at different time intervals by others.
One known solution, referred to in the Finnish Patent Application no. 81 3502 and the U.S. Pat. No. 3,758,849, requires the use of two pairs of transmitter and receiver coils, each one consisting of two superimposed windings, equal and crossed. In the solution as constructed in the Finnish patent application no. 81 3502, single metal bodies in transit, especially long, thin structures, positioned in the direction of one of the two pairs of windings, are automatically perpendicular to the other, and therefore always give a substantial signal on at least one of the receiver windings.
This solution for reasons of symmetry, is not ideal for improving the conditions for detecting bodies like those mentioned above, that are passing through the areas of minimum field inside the supervised passageway. This drawback is overcome with the invention that is the subject of Italian Patent Application no. AR 91 A 000015 in the name of Alessandro Manneschi which, due to the symmetrical staggering in height of the transmitter and receiver windings, it is possible to compensate for the minimum signals of the magnetic field of one winding with the largest signal of the other and, in particular, compensate for the minimum field effect on the ground and at the top of the supervised passageway.
However, existing solutions making use of multipolar windings, including those shown above, have one serious drawback that derive from the fact that the simultaneous transit of two or more metal bodies, under certain conditions, can give a reception signal that is far inferior to that generated by the passage of a single one without triggering the alarm. This occurs, for example, whenever two metal bodies pass through simultaneously both maintaining the same position and distance so that,--depending on the structure of the windings,--each produces a variation in incoming signals that is opposite to the other.
Given that in multipolar windings, if they are in a series of alternating opposite poles, the resulting E.M.F. variation in the whole winding terminal, following the simultaneous passage of two bodies that correspond to opposite poles, is minimized, or even nullified.
The same phenomenum occurs again when more than one metal body is in simultaneous transit with a reciprocal position so that each body produces a variation in signal, the algebraic sum of which is small.
Therefore, an adjustment of sensitivity, programmed to reveal a single body, is not sufficient, as in the case shown, to intercept the simultaneous passage of two or more bodies.
Even more serious is the fact that, with known metal detector sensory systems, once the reciprocal position needed to give the above described effect has been found out, by keeping this distance between the bodies and moving them as they advance on a vertical plane through the passageway, the resulting effect of minimization is substantially maintained.
It is clear, therefore, that there is an innumerable series of transit routes, moving on a vertical plane which cause the resulting signal provoked by the group of bodies to be diminished, and for this reason, once these routes which give the minimum signal on the windings have been discovered along with the distance at which these bodies should be kept apart, their transit through the corridor controlled by the Metal Detector is guaranteed not to be registered. Consequently existing metal detectors with one or two pairs of basically equal windings crossed over each other, are able to detect the passage of a single metal body whose mass exceeds a calibrated value, but become vulnerable when two metal bodies with equal features pass through simultaneously at a distance that provokes a minimum signal, hereafter called "pitch".